1. Field of Invention
The present invention relates to a stamper for fabrication of an optical recording medium, a method of forming an information recording area and a light transmissive layer using the stamper, and an optical recording medium.
2. Description of Related Art
Optical discs on which information signals are written by embossed pits are widely used as a form of optical recording media for audio and video, and for recording other various information. The basic form of optical disc is shown in FIG. 11. This optical disc 2 comprises an information recording layer 8 having a reflective film 6 on a light transmissive molded substrate (supporting plate) 4. A protective layer 12 made of an UV curable resin or the like is formed thereon. In this optical disc 2, a laser beam La is irradiated on the side of the molded substrate 4 acting as a light transmissive layer so as to read an information signal from the information recording layer 8.
A recording density of the optical disc 2 can be increased as a spot diameter of the used laser beam La is reduced. The spot diameter is proportional to: λ/NA (λ: a wavelength of a laser beam, NA: a numerical aperture of an objective lens). Therefore, in order to reduce the spot diameter, it is necessary either to reduce the wavelength λ of the laser beam La or to increase the numerical aperture NA of the objective lens.
With respect to the wavelength λ of the laser beam La, a wavelength of 780 nm is used conventionally for CDs, whereas a wavelength of 650 nm is conventionally used for DVDs. A wavelength in the vicinity of 400 nm corresponding to a blue laser is expected for use in the future. Since the spot diameter is reduced for the decreased wavelength, a recording density is increased.
With respect to numerical aperture NA, coma aberration is increased as the numerical aperture NA is increased. In order to keep coma aberration small even with the increased numerical aperture NA, it is necessary to reduce a thickness of the light transmissive layer (the molded substrate 4) through which the laser beam La passes.
Conventionally, a molded product made of a resin, molded by injection molding or the like, is frequently used as the light transmissive layer (the molded substrate 4) of the optical disc 2. However, it is difficult to fabricate the molded substrate 4 to have a smaller thickness than that of currently used DVDs with good accuracy using this current technique. Specifically, in view of this current fabrication technique, the ability to increase a recording density by an increased NA in the optical disc 2 having a structure using the molded substrate 4 as a light transmissive layer, as shown in FIG. 11, has reached the acceptable limits.
To cope with this, a technique for achieving a further increase in an NA of laser beam by forming a light transmissive layer has been proposed in, for example, Japanese Patent Laid-Open Publication No. 1996-235638, Japanese Patent Laid-Open Publication No. 1999-203724, and the like.
Specifically, this technique as shown in FIG. 12, includes forming a molded substrate 50 that is not required to transmit any light. The substrate 50 is formed by injection molding to have a large thickness as a base of a disc body. Next, an information recording layer 54 having a reflective layer 52 is formed on the molded substrate 50. An extremely thin (at the maximum, about 300 μm) light transmissive layer 56 is deposited thereon by means, such as attachment of a light transmissive sheet, resin coating through spin coating, or the like.
The laser beam La is irradiated on the side of the thin light transmissive layer 56 to record and/or read information on and/or from the information recording layer 54. In this manner, it is possible to form the thinner light transmissive layer 56 while ensuring the strength of the disk due to the molded substrate 50. Therefore, a higher density (larger capacity) can be achieved by using a higher NA of the laser beam.
Under actual conditions, however, the above-described technique still has various problems with respect to forming the thin light transmissive layer.
Generally, as a method for forming the thin light transmissive layer, a method of bonding a transparent sheet onto a molded substrate (or a supporting plate), a method of applying (coating) a resin by spin coating, and the like have been proposed. However, the method of bonding a transparent sheet is likely to have elevated fabrication cost because strict optical characteristics of the sheet and accuracy in its thickness are required. Moreover, even if the thickness of the transparent sheet is accurate, an uneven thickness or uneven application of an adhesive may still change the optical characteristics of the layer. Thus, high accuracy is required even for the application of an adhesive. Accordingly, under the actual conditions, many problems remain in this method.
The method of forming a thin light transmissive layer by application of a resin through spin coating specifically consists of making a liquid UV curable resin (applied liquid) flow on a surface of a molded substrate (supporting plate) while rotating the molded substrate with a spindle so that the resin is uniformly spread in accordance with the rotation of the molded substrate to coat the resin layer. Thereafter, the coated resin is cured by irradiation with ultraviolet radiation. According to this method, various conditions such as a thickness of the coating can be adjusted by controlling a rotation speed of the spindle, application time, and a viscosity of the applied liquid.
However, this method has a problem in that the flatness of the surface of the coating film is difficult to maintain. A concentration (raise) of the application liquid is likely to occur, in particular, in the vicinity of the outer periphery. The raise does not become such a serious problem in the case where there is a relatively large distance between a laser optical system and a disc such as in a conventional CD and DVD. However, under the conditions where a gap between a laser optical system and a disc is reduced, such as shown in FIG. 12, it becomes problematic.
In particular, in the case where a relatively thick (for example, 100 μm) coating layer is formed, a thickness or a radial width of the raise may become too large in an information recording area. Therefore, if no countermeasure is taken, there is a possibility that the raise will reduce a recordable area in the vicinity of the outer periphery so that the recording/reading of information signals to be obtained may be impaired. Moreover, the raise may also cause the collision of a laser optical system against a disc.
Regarding a treatment of the raise or the maintenance of smoothness of the coating surface, various proposals for ways of coping with them have been made in, for example, Japanese Patent Laid-Open Publication Nos. 1999-203724, 1999-86355, 1999-86356, and the like. However, these methods merely fix the problem after it has occurred rather than preventing it. Therefore, the fabrication process is complicated and fabrication costs are increased.